In the processing of a semiconductor wafer to form integrated circuits, it is common to etch holes through one or more layers of materials formed on the wafer to provide access to underlying materials. Typically, this is done to permit conductive materials to be deposited in these holes for the purpose of creating contacts for external connections circuits or conductive "vias" between different internal wiring layers. Via holes provide electrical access to underlying metal conductive layers (wiring layers) while contact holes provide electrical access to underlying silicon or polysilicon (e.g., source or drain diffusion regions or gate polysilicon). In the process of etching these holes, formations called "fences" are known to be created on vertical surfaces (e.g., the "walls" of the holes). These fences are formed of a carbon-fluorine-silicon-oxygen polymer that is a natural result of the etching process and which is deposited evenly across the wafer. On horizontal surfaces, the polymer is removed as fast as it is deposited by ion bombardment that occurs on those surfaces during etching. On vertical surfaces, however, the polymer builds up creating "fences".
If fences are left on the wafer, it can be difficult, if not impossible, to create good contacts or vias due to poor adhesion of the contact material, contamination, or step coverage problems (i.e., inability to completely fill the holes with conductive material) leading to high contact resistance with the underlying material. Typically, fences are removed with organic solvents (such as NMP) prior to depositing conductive materials.
During the etching of contact holes to underlying silicon or polysilicon, the "fence" polymer has the beneficial side-effect of protecting silicon and polysilicon materials (commonly used in silicon-based integrated circuit structures) from being attacked by the etching process. The polymer has no beneficial effect when via holes are etched to underlying metal.
In modern integrated circuit devices, aluminum (chemical symbol "Al") is commonly used as an internal interconnect (wiring) material and for contacts. Titanium and titanium compounds, such as titanium nitride (hereinafter referred to by its chemical formula "TIN") are used as an antireflective coating over aluminum, as a diffusion barrier under aluminum, and as an adhesion layer under tungsten (chemical symbol "W"), which is often used to fill contact and via holes. Small geometries of modern integrated circuits have necessitated the use of "dull" antireflective materials, such as TiN, over shiny materials, such as aluminum, to prevent reflections off of the shiny materials from affecting "photo" processes.
Hereinafter, titanium and titanium compounds will be referred to collectively as "titanium-containing materials", and will be understood to encompass both elemental titanium and titanium compounds.
In the process of etching contact or via holes (e.g., by plasma etching, well known to those skilled in the art), if Al is exposed to the etching process, it may be rapidly "sputtered" about and becomes incorporated into the fence polymer. This makes the fence polymer difficult to remove effectively, since removal of an Al-tainted fence requires the use of solvents which also attack the aluminum wiring material. A similar effect can occur if a titanium-containing material is encountered during the etching process, similarly causing titanium to be incorporated into the fence polymer, creating a Ti-tainted fence, which is equally difficult to remove effectively. (It is also possible to incorporate both titanium and aluminum into a fence.)
In integrated circuits where TiN is used, it is often desirable to etch away just the TiN in a selective etch process. Three techniques of etching TiN are known in the prior art:
Etching in a fluorine plasma with high ion bombardment, PA0 Etching in a chlorine plasma PA0 Wet etching
Wet etching may remove thin films of TiN, but cannot be used in small contact holes or vias. Surface tension in combination with the small size of the holes prevents the etchant from reliably wetting the bottom of the contact holes or vias. A vacuum method of etching is preferable, to help ensure contact of the TiN film with the etchant.
Fluorine plasma for etching with high ion bombardment is most commonly used as an over-etch step for etching vias and contact holes. This method has the following problems:
The etch rate of the TiN is very slow (10% of the etch rate of SiO.sub.2). The etch process removes photoresist as well as titanium-containing materials, exposing underlying materials to the etch process. If the photoresist is completely removed, oxide (SiO.sub.2) loss from the top surface of the wafer and alteration of the profile of the via or contact hole are likely.
A polymer is formed in the via (or contact) hole consisting of titanium, aluminum, oxygen, silicon, and carbon. The polymer must be removed to ensure device reliability, but is extremely difficult to remove selectively without causing damage to or erosion of other materials.
A chlorine plasma environment may also be used to etch titanium-containing materials, but after the titanium-containing materials are removed, aluminum may be exposed to the plasma. In this case, chlorine is incorporated in the aluminum and causes undesired aluminum etching or corrosion.
FIG. 1a shows a semiconductor device 100 having an upper surface formed by a layer of insulating material (usually SiO.sub.2 ) into which a via hole 110 has been etched by a process such as plasma etching. Underlying the insulating material 102 is an aluminum conductive wiring layer 104, over which an anti-reflective film of titanium-containing material 112 has been deposited. Underlying the aluminum wiring layer 104 is another insulating layer 106. A polymer 108 is shown on the walls of hole 110 as a result of the etching process. Ordinarily, titanium-containing materials are sufficiently conductive to provide good electrical contact, but damage 120 done to the surface titanium-containing material during the process of etching hole 110 would cause poor contact between any conductive material deposited in hole 110 and aluminum 104 it the damaged titanium material 120 were allowed to remain. Therefore, it is desirable to remove the damaged titanium material 120 prior to depositing any conductive material in the via hole.
FIG. 1b illustrates a prior-art fluorine plasma etching process 116 applied to wafer 100 to remove .the damaged titanium material 120 of FIG. 1b. (It is assumed that there is a photoresist applied to the top surface of the wafer to mask off the via hole prior to etching, but since the photoresist is attacked and removed by the fluorine plasma etching process, it is not shown). After removal of the damaged titanium material 120, aluminum 104 is exposed in the area shown by 114, and is rapidly sputtered and incorporated into an aluminum containing polymer 108'. (Titanium is also incorporated into the polymer as the damaged titanium material 120 is etched). The overlying insulating layer 102 is eroded by the etching process 116 once any photoresist is gone, causing oxide loss resulting in a thinned insulating layer 102'. The aluminum-containing polymer is extremely difficult to remove without causing significant further damage to aluminum 104 in the area shown by 114.
FIG. 1c shows another wafer 130 having a lower insulating layer 136, an aluminum conductive wiring layer 134 overlying layer 136, and an upper insulating layer 132 overlying layers 134 and 136. A hole 140 has been etched into upper insulating layer 132 by a suitable etching process. A film of a titanium-containing material 142 has been deposited over the upper insulating layer 132, completely covering layer 132 and the surface of hole 140. The titanium-containing material 142 provides an adhesion layer for a tungsten (W) plug 144 which has been deposited into hole 140. Titanium-containing material 142 is sufficiently conductive and sufficiently thin that good electrical contact is provided between aluminum 134 and tungsten 144. However, a thickness "t" of titanium containing material 142 overlying the top surface of upper insulating layer 132 effectively "shorts out" any other similarly deposited tungsten plugs, and must be removed.
FIG. 1d shows the same wafer 130' after removal of the thickness "t" of titanium-containing material. Because prior-art techniques of removing titanium-containing materials do not provide good control of the degree of etch, it is possible titanium-containing material 142 (with respect to FIG. 1c) to be removed around the sides of tungsten plug 144, leaving the shape shown as 142'. This causes undesirable trenches 143 around the tungsten plug 144.
FIGS. 1a-1d all show via holes (i.e., holes which make contact with an underlying wiring layer. The discussion hereinabove, however, is also applicable to contact holes.
FIG. 1e shows a wafer 150 wherein a contact hole 160 is etched into an oxide layer 152 to provide electrical access to a silicon n-diffusion region 154 in a p-material 156 underlying oxide layer 152. A thin coating of a titanium-containing material 162, such as titanium nitride, is applied over the oxide layer 152 and hole 160 as an adhesion layer for a tungsten plug 164 deposited into hole 160 over the layer 162 of titanium-containing material. The problem of etching the titanium layer 162 is essentially identical to that of FIGS. 1c and 1d.